In almost all cases today, coils for electrotechnical applications are wound during mass production. Because the wound enamel-insulated wires can exhibit defects, the occurrence of an interturn or even an interlayer fault cannot be overlooked. Interturn faults remaining undetected in the course of production can possibly cause long-term coil failure. Therefore, within the scope of quality assurance, it is necessary for the ready-wound coils to undergo a test for interturn and/or interlayer faults. Several measuring methods, following very diverse physical principles, can be used for this purpose.
In previous practice, customary measuring methods operate, for example, according to the principles of an a.c. voltage measuring bridge or an impulse voltage testing method. Measuring instruments produced according to these principles operate satisfactorily when the coils have only a small number of turns. However, when the coils have a larger number of turns, the prior known instruments prove unreliable due to the s parasitic coil capacitance or the minimal measuring effect caused by an interturn fault.
Due to the fact that increasingly higher demands are placed on coils, any interturn faults across a turn must be reliably detected. This is particularly true for coils having a very high number of turns. The measuring method must also be suitable for application in a flexible, large-scale coil production.
Prior known German Patent 910 092 discloses a device for testing short-circuited turns in a coil. The device includes two auxiliary coils, one of which is coupled to an alternating-current source and the other to a voltmeter. The auxiliary coils are magnetically coupled, preferably through two iron cores, to -he coil to be tested. One auxiliary coil hereto operates with industrial alternating current (50 Hz), whereby the voltmeter is supposed to be calibrated directly in the short-circuited turns.
Another known device for testing a winding for interturn or interlayer faults is described in German Printed Patent 26 44 253. This device discloses permitting an accurate testing for interturn or interlayer faults. The device is operable for windings having a very high number of turns. The device includes a first U-shaped core having an excitation winding around one of its arms. An additional U-shaped core has a measuring winding around one of its arms which is coupled to a display device. The winding to be tested can be wrapped around the adjacent other arm of both U-shaped cores. In a first specific embodiment, an auxiliary winding is superimposed on both U-shaped cores. An adjusting device, which is adjustable with respect to the phase relation of a delivered current, is coupled to the auxiliary winding. In this embodiment, the current must be manually adjusted in proportion to the test piece such that for a standard winding, the display device shows the value zero. In a second specific embodiment, the auxiliary winding is dispensed with, provided that the measuring device itself is phase-sensitive. In both cases, however, it is necessary to select the lowest possible measuring frequency to guarantee adequate specificity.
In the German Printed Patent 26 44 253, the current adjustment depends on the type of coil and thus essentially can only be performed manually. The accuracy of the adjustment thereby influences the measuring result. As a result, in practice, more work and control are required to perform the test.
There is therefore needed a method and corresponding device for performing the winding test without requiring any manual adjustment steps, or alternatively where a phase-sensitive measuring device is not required.